Oil Temp Problem

Hi All

I had 15w-50 oil in my engine but due to me doing short distance driving oilman recommended i use 5w-40 so i did, but the problem i have is it takes the 5w-40 fully synthetic the same amount of time as the 15w-50 to reach 80 degrees?? I also have a problem when im driving along all of a sudden the temp would be on 80 degrees then drop to 70 and back up to 80 but most of the time it drops down to 70 why is that does anyone know?

Thanks

Mark

Does it drop slowly to 70 or one big sudden drop then increase again?

Do you have an oil PRESSURE gauge?

[quote author=AndyC link=topic=27778.msg324733#msg324733 date=1290597508]
Does it drop slowly to 70 or one big sudden drop then increase again?

Do you have an oil PRESSURE gauge?
[/quote]

Its a sudden drop to 70 and then gradually up to 80 again. Oil pressure gauge stays at 6 bar while driving along at 100kms.

If pressure gauge is still reading normal then I'd say its a problem with the temp gauge itself or the temp sensor.

i would put my money on a dodgy sensor

I checked the wiring and its okay, im buying a new gauge with sensor.. hopefully it will be okay again. if not would there be a problem with a valve or something wrong with the oil cooler?